Calculating the Relative Humidity In Your Home
Here's how we check the approximate relative humidity in our loft.
To begin with we need the current outdoor temperature and the current dew point which we get from any number of weather sites. In this example they are 0 and -6 respectively.
We load the calculator (http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html), plug those numbers into the appropriate boxes and hit the "Calculate" button. Hopefully the resulting RH (relative humidity) will be close to the same as shown on the weather page we visited earlier.
Next we clear the RH field, replace the temperature field with our current indoor temperature and calculate again. In this example, with no additional humidification the indoor RH would be a staggering 50% less.
In all honesty, we never actually do any of those calculations for our location. We have a bunch of hygrometers scattered around the loft and one or two full room humidifiers running from late September until well into May. We make sure we're at at least 40% RH but it's good to know just how much of an uphill fight it is to maintain comfortable and safe mumidity. Try it for your home.